Powder metal technology is well known to the persons skilled in the art and generally comprises the formation of metal powders which are compacted and then subjected to an elevated temperature so as to produce a sintered product.
Various processes have heretofore been designed in order to produce sintered articles having high densities. Such processes include a double press double sintering process for densities typically up to 7.5 g/cc as well as hot powder forging where virtually full densities of up to 7.8 g/cc may be obtained. However, such prior art processes are relatively expensive and time consuming. Recently developed methods include warm pressing of powders up to 7.35 g/cc as disclosed in U.S. Pat. No. 5,154,881. However, there are process disadvantages with the warm pressing such as maintaining tool clearances with heated systems. Also warm pressing does not allow very high densities up to above 7.5 g/cc to be easily reached in commonly used alloy systems without double pressing and double sintering.
Moreover, U.S. Pat. No. 5,009,842 refers to a hot forging operation to be carried out on a sintered part after quenching and after pre-heating the part to at least 1,000.degree. C. and then subjecting the pre-heated sintered part to impact. Furthermore, U.S. Pat. No. 3,901,961 illustrates a pre-alloyed steel powder for formation of structural parts by powder forging and powder forged articles for structural parts.
U.S. Pat. No. 4,014,680 teaches pre-alloyed stainless steel powder for liquid phase sintering, while U.S. Pat. No. 4,069,044 illustrates a method of producing forged articles from prealloyed-premixed water atomized ferrous alloy powder.
Moreover, R. Laag et al, in an article entitled "Super Plastic Forming of Ultrahigh Carbon Alloyed PIM Steels", page 409-421 relates to super plastic forming for the production of net-shaped parts produced by inert gas, atomization and Hot Isostatic Pressing, Osprey processing or thermal mechanical treatment of casting alloys.
It is an object of this invention to provide an improved process for producing ultra-high carbon steels having improved dynamic strength characteristics and an accurate method to control the size.
It is a further object of this invention to provide an improved process for producing sintered articles having improved strength characteristics with carbon content between 0.8% to 2.0% carbon which have been spheroidized and an accurate method to control same.
The broadest aspect of this invention relates to a method of making a sintered article of powder metal having a carbon composition in the range of about 0.8% to 2.0% by weight, then spheroidizing said sintered article and then warm forming said sintered article at a temperature between 250.degree. C. and 700.degree. C. for a time duration selected to accurately form said article to a final shape.